Ising-Induced Spectral Broadening Resolves the Relaxation Bottleneck in Superradiant Masers
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Abstract
The recent observation of self-induced superradiant masing [[W. Kersten et al., Nat. Phys. 22, 158 (2026)]] revealed a collective relaxation timescale significantly slower than predicted by standard coherent transport models. Here, we elucidate the microscopic origin of this ``relaxation bottleneck.'' We show that in the high-density regime relevant to the experiment, diagonal Ising interactions -- often treated as perturbative -- generate profound inhomogeneous broadening that exceeds the intrinsic single-particle dephasing. This intense diagonal disorder suppresses resonant flip-flop exchange, effectively renormalizing the density of states available for spectral diffusion. Our parameter-free analytic theory quantitatively reproduces the experimentally observed microsecond dynamics, identifying Ising-induced broadening as the governing mechanism for energy transport in dense solid-state spin ensembles.